Many different types of printing have been developed, including a large number of processes which are presently in use. The known forms of printing employ a variety of methods for printing onto a substrate. Commonly used forms of printing include offset printing, laser printing, copying devices, dot matrix type impact printers, thermal paper printers, film recorders, thermal wax printers, dye sublimation printers, and inkjet printers. Each type of printer has its own advantages and problems when considering cost, speed, quality, reliability, and simplicity of construction and operation.
Electrostatic printing is a very effective method of image transfer commonly used in photocopying and photoprinting. Typically, in electrostatic printing, a potential electrostatic image is formed on an imaging surface carrying a uniform electrostatic charge. The uniform electrostatic charge can be created by exposing the surface to corona discharge. The uniform electrostatic charge is then selectively discharged by exposing it to a modulated beam of light which corresponds to an image formed from an original. The discharged surfaces form the background while the charged surfaces form the print image. The print image is developed by applying pigmented toner particles which adhere to the undischarged “print” portions of the surface. The pigment is subsequently transferred by various techniques to a copy sheet.
Dry toner is most commonly used in electrostatic printing. The quality and clarity of the image and image resolution is related to the size of the toner particles. While it is thought that very fine particles will produce a finer image, there is a practical limitation on the size of toner particles that can be used. Dry toner particles must be of sufficient weight and size to be deposited onto the print surface without becoming airborne, which is thought to lead to machinery fouling and, possibly, environmental problems. Additionally, in fixing the image, the dry toner particles are fused onto the paper by exposure to very high temperatures, e.g. in excess of about 400° F. (204° C.). This energy requirement is a significant drawback.
To overcome these disadvantages, liquid toners were developed in which the toner is dispersed in a solvent. The solvent is removed in the last printing step by the mechanism of the press. Because of the liquid medium, very fine dye particles can be employed without concern for the particles becoming airborne. Thus, copies of very high resolution can be made and high temperatures needed to fuse dry toners are not required. Liquid toners for electrostatic imaging are described in U.S. Pat. Nos. 5,225,306; 5,276,492; 5,346,796; and 5,407,771.
Paper is widely used as the image-receiving element in electrostatic imaging. It would be advantageous to use plastic as the receiving element. Among other advantages over paper, plastic is moisture resistant, flexible, and heat sealable and plastic substrates can be either clear or opaque. However, the high temperatures necessary for imaging with the dry toners will melt plastic films and the liquid toners do not transfer well or adhere to uncoated plastic.
U.S. Pat. No. 5,789,123 to Cleckner et al. discloses a liquid toner printable thermoplastic film. The film is coated with an ethylene-acrylic acid copolymer based coating capable of electrostatic imaging with liquid toner. Optionally, the coating contains acrylic polymer. In a specific embodiment, the coating includes a major proportion of ethylene-acrylic acid and minor amounts of filler such as talc and silica. The coating can also include wax and/or pigment such as titanium dioxide. In a further embodiment, the carboxylate groups of the copolymer are neutralized with metal ions from Group Ia, Ia or IIb of the Periodic Table of the Elements, specifically, sodium.
In recent years, the field of inkjet printing, wherein each individual pixel of ink is derived from one or more ink nozzles, has become increasingly popular primarily due to its inexpensive and versatile nature. In a typical inkjet recording or printing system, ink droplets are ejected from a nozzle at high speed towards a recording element or medium to produce an image on the medium. The ink droplets, or recording liquid, generally comprise a recording agent, such as a dye or pigment, and a large amount of solvent. The solvent, or carrier liquid, typically is made up of water, an organic material such as a monohydric alcohol, a polyhydric alcohol, or mixtures thereof.
An inkjet recording element typically comprises a support having on at least one surface thereof an ink-receiving or image-forming layer, and includes those intended for reflection viewing, which have an opaque support, and those intended for viewing by transmitted light, which have a transparent support.
While a wide variety of different types of image-recording elements for use with inkjet devices have been proposed heretofore, there are many unsolved problems in the art and many deficiencies in the known products which have limited their commercial usefulness.
For example, it is well known that in order to achieve and maintain photographic-quality images on such an image-recording element, an inkjet recording element must: (a) be readily wetted so there is no puddling, i.e., coalescence of adjacent ink dots, which leads to non-uniform density, (b) exhibit no image bleeding, (c) absorb high concentrations of ink and dry quickly to avoid elements blocking together when stacked against subsequent prints or other surfaces, (d) exhibit no discontinuities or defects due to interactions between the support and/or layers, such as cracking, repellencies, comb lines, and the like, (e) not allow unabsorbed dyes to aggregate at the free surface causing dye crystallization, which results in bloom or bronzing effects in the imaged areas, and (f) have an optimized image fastness to avoid fade from contact with water or radiation by daylight, tungsten light, or fluorescent light.
An ink jet recording element that simultaneously provides an almost instantaneous ink dry time and good image quality is desirable. However, given the wide range of ink compositions and ink volumes that a recording element needs to accommodate, these requirements of inkjet recording media are difficult to achieve simultaneously.
The rise of radiation-curable, especially UV curable, inks in inkjet printing processes are highly desirable since, after appropriate curing, radiation curable inks provide a tough, durable image upon the substrate to which they are applied. This makes the process especially applicable to printing on plastics packaging where high durability is required. Therefore, it can be applied not only to conventional cellulosic substrates such as paper and board, but also to synthetic polymeric substrates.
As is commonly known in the art, the addition of filler(s) to a polymeric film substrate serves to improve the strength and permanence of the film. In printing applications, filler(s) may also be used to absorb excess ink and solvent to enhance print quality. In the prior art, large quantities of filler, for example greater than 5.0 wt %, are typically employed in the substrate composition to promote these qualities.
U.S. Patent Application Publication US-2001/0009701-A1 discloses UV curing printing inks having a UV curable fixing agent system comprising a polymerizing fixing agent or a mixture of fixing agents and one or more associated photo-initiators. The polymerization or cross-linking can be triggered by UV irradiation to cure the ink. Publication US-2001/0009701-A1 differentiates between radical-induced and cationic polymerization. Conventional radical-induced polymerizing fixing agents are based on acrylates, whereas cationic polymerizing fixing agents are characterized by acid release during UV irradiation. Publication US-2001/0009701-A1 also notes that UV curing printing inks have practical advantages from a technical applications point of view compared to solvent-containing inks, e.g., with regard to their working lifetime, solvent related environmental pollution, and waste disposal.
As is commonly known in the art, the use of fillers in polymeric coatings for UV ink jet printing applications increases both the surface area and the density of the polymer. However, the use of fillers in these applications is also known to negatively impact yield (i.e., film surface area per weight unit of the thermoplastic film). Accordingly, there is a need for a thermoplastic film capable of receiving UV curable printing inks which provide satisfactory ink adhesion and high print quality while employing reduced amounts of filler in the film composition to increase overall yield.